The well-known Bayer process involves the manufacture of alumina trihydrate from bauxite by subjecting bauxite in a caustic liquor to digestion at elevated temperatures and pressures. In the digestion step the caustic liquor dissolves the alumina content of the bauxite and a caustic aluminate liquor is obtained together with a caustic-insoluble residue, the so-called "red mud". From the caustic aluminate liquor the alumina content is generally recovered by precipitation.
Most bauxites contain organic matters the quantity of which usually varies with the source of bauxite. The organic matter, which consist of a wide variety of organic compounds of varying molecular weight, is also dissolved by the caustic liquor during digestion and will become part of the alkaline aluminate liquor. The Bayer process is a cyclic process, i.e. the caustic liquor, after precipitation of the alumina content, is recycled to the digestion of a new bauxite charge, and as a result, the dissolved organics accumulate in the caustic liquor. After extended operation of the Bayer process the concentration of the organic compounds increases to a harmful level resulting in decreased alumina trihydrate yields, decreased red mud settling rates and other operational problems. These problems include premature nucleation during alumina trihydrate precipitation and liquor foaming. As mentioned before, the organic compounds dissolved in the liquor vary in molecular weight. These organics can be present as the alkaline salts of lower carboxylic acids (formate, oxalate, acetate) and include salts having seventy or more carbon atoms. These high molecular weight organics are mostly humates derived from humic acids. These humates are a small part of the total dissolved organic content of the liquor but they are considered the most detrimental to the efficient and economical operation of a Bayer process plant. In particular, these humates, apart from their color-forming property, indirectly induce undesired nucleation of saturated alkali aluminate solutions and result in the formation of a large percentage of small (&lt;45.mu.size) alumina trihydrate particles, cause foaming and in general interfere with the effective operation of Bayer process plants, particularly in classifying hydrate particles.
The harmful properties of humates has already been recognized and several methods are being offered to at least reduce their undesirable effect on the Bayer process. U.S. Pat. No. 4,046,855 (Schepers et al) discloses a process wherein organic impurities can be removed from Bayer process liquors by contacting the liquor with a magnesium compound which will form a precipitated mixture of magnesium and aluminum hydroxides. This precipitate removes some of the organic impurities either by adsorption or by chemisorption. The magnesium compound may be added at any stage of the Bayer process, additions prior to digestion or to the digested slurry are preferred. Although this process is capable of removing at least a portion of the organic impurities, the formation of a precipitated hydroxide mixture creates operational difficulties. The precipitated hydroxide mixture will contain aluminum hydroxide and this results in product alumina loss. Also, the precipitated mixture has to be separated from the rest of the treated liquor and this involves additional processing steps and/or a definite increase in the quantity of the total mud load which has to be disposed of. In U.S. Pat. No. 4,101,629 (Mercier et al), a barium-containing compound is added to Bayer process liquors. The barium compound precipitates as barium aluminate and the precipitated material may also include barium salts of organic impurities present in the liquor. This process also involves precipitation of a compound which has to be removed from the treated liquor thus requiring settling and/or filtration equipment and additional processing steps. The process allows recovery and reuse of the filtered barium compound by calcination; however, the well-known toxicity of barium salts may create an unacceptable environmental and/or health risk not justifiable by the purification results obtainable by it. Other patents, relating to the incorporation of additives in Bayer process liquors for the removal of organics, include U.S. Pat. Nos. 4,275,042, 4,275,043, 3,832,442.
Another method of eliminating dissolved organic compounds from Bayer process liquors involves the oxidation of the organics using oxygen or an oxygen-containing gas. In United Kingdom Patent Application No. 2,037,722 (published July 16, 1980), organic compounds are removed from Bayer process liquors by introducing large quantities of oxygen into the liquor kept at 120.degree.-350.degree. C. until a partial oxygen pressure of 3-30 atmospheres is attained. As the oxidation proceeds additional oxygen or air is introduced into the pressurized system. The method can achieve total oxidation of the organic content of the liquor during the digestion step at the recommended 210.degree.-300.degree. C. temperature and the solid residue of the oxidation, mainly sodium carbonate, will be removed together with the red mud. While this process destroys organic contaminants including the humates in the liquor, it is associated with certain operational and mechanical difficulties which render this total oxidation method unattractive. These involve the use of large volumes of oxygen and even larger volumes of oxygen-containing gas, such as air, and the inherent difficulty of introducing such large volumes into a pressurized, high temperature system. Additionally, the process forms a relatively large quantity of solids which tend to scale the equipment used. Oxidation of a large proportion of total organic content results in the formation of large quantities of oxalate and in the excessive carbonation of the process liquor, both of which are highly undesirable. Also, oxidation of the entire organic contaminant content is a time-consuming reaction and, as shown, it requires 3-60 minutes to destroy 20-30% of the organics at 180.degree. C. For better oxidation results higher termperatures are required, for example at 260.degree.-280.degree. C. about 97% oxidation can be achieved. Since most Bayer process digestions are accomplished at temperatures below about 250.degree. C., it becomes evident that to accomplish complete oxidation at the usual digestion temperatures either the time period of digestion has to be significantly lengthened or the oxygen charge to the digestion has to be substantially increased, or both of these alternatives must be applied to obtain satisfactory results.
In U.S. Pat. No. 4,215,094 it is recommended that the oxidation of organics present in the liquor be accomplished at 180.degree.-300.degree. C. and at 20-150 kg/cm.sup.2 pressure (19.3-145 atm) in the presence of a copper salt catalyst which is added in an amount equal to 300-5,000 g/L copper ion. Regardless of the catalyst addition and the high pressure and temperature applied, the best oxidation results shown at the usual digestion temperatures are in the range of 10-48%. In addition, the copper catalyst has to be removed from the treated liquor to avoid contamination. Disposal of the removed copper catalyst can create environmental and/or health hazards.
The instant process, which is directed to the removal of the humate content of the contaminated Bayer process liquor, avoids all of these aforementioned difficulties. The desired humate destruction is accomplished by introducing in the Bayer process liquor only as much oxygen as is soluble in the liquor under the particular processing conditions. This avoids the use of temperatures which are above the conventionally employed range, avoids the use of catalysts and large quantities of oxygen or air, accomplishes humate destruction well within the time available in the digestion stage of the Bayer process and makes use of existing process equipment.